This project contributes to our overall objective of exploring novel cellular and gene therapy approaches for HIV-1-infected human infants by focusing on the stromal aberrations associated with pediatric simian immunodeficiency virus (SIV) infection, and strategies for transplanting autologous marrow stromal cells genetically engineered to resist SIV infection. These studies will provide essential information on the mechanism(s) responsible for SIV-induced hematopoietic suppression, the role of marrow stromal cells, and methods for providing the necessary supportive elements to reverse SIV-induced hematopoietic abnormalities. Our first goal (Specific Aim 1) is to confirm in vitro that infection of rhesus marrow stroma with pathogenic SIV results in hematopoietic suppression, similar to findings with HIV-1. Here, we will study the effects of pathogenic SIV on the bone marrow microenvironment by examining stromal cell monolayers in long-term bone marrow culture (LTBMC). We will investigate stroma genetically altered to resist SIV replication using the LTBMC. These studies will show in vitro that stromal SIV replication is the essential component responsible for decreased hematopoietic cell production with pathogenic SIV infection comparable to findings with HIV-1, and support studies focusing on the transplantation of genetically-altered stromal cells capable of resisting SIV replication. Specific Aim 2 will focus on engraftment efficiency of autologous stromal cells in uninfected and SIV-infected infant rhesus monkeys. We will study engraftment of genetically-marked autologous stromal cells, identify the best cell concentration for transplant, and determine whether engraftment efficiency is improved with an intra-bone marrow (IBM) versus intravenous (IV) approach. In Specific Aim 3, we will transplant transduced stromal cells capable of resisting SIV infection. We will collect bone marrow from newborn monkeys (uninfected, SIV-infected), expand the stromal cells in culture, transduce stromal cells to resist SIV infection using anti-SIV genes (see Specific Aim 1), then transplant transduced stromal cells using the best approaches identified in Specific Aim 2. These studies will allow us to test the hypothesis that engraftment of transduced marrow stromal cells is sufficient to inhibit SIV replication and subsequent hematopoietic suppression.